Merge pull request #3941 from google/anandolee-patch-2

Add _file_desc_by_toplevel_extension back

python/google/protobuf/descriptor_pool.py

@@ -127,6 +127,9 @@ class DescriptorPool(object):
     self._service_descriptors = {}
     self._file_descriptors = {}
     self._toplevel_extensions = {}
+    # TODO(jieluo): Remove _file_desc_by_toplevel_extension after
+    # maybe year 2020 for compatibility issue (with 3.4.1 only).
+    self._file_desc_by_toplevel_extension = {}
     # We store extensions in two two-level mappings: The first key is the
     # descriptor of the message being extended, the second key is the extension
     # full name or its tag number.
@@ -252,6 +255,12 @@ class DescriptorPool(object):
     """
 
     self._AddFileDescriptor(file_desc)
+    # TODO(jieluo): This is a temporary solution for FieldDescriptor.file.
+    # FieldDescriptor.file is added in code gen. Remove this solution after
+    # maybe 2020 for compatibility reason (with 3.4.1 only).
+    for extension in file_desc.extensions_by_name.values():
+      self._file_desc_by_toplevel_extension[
+          extension.full_name] = file_desc
 
   def _AddFileDescriptor(self, file_desc):
     """Adds a FileDescriptor to the pool, non-recursively.
@@ -331,7 +340,7 @@ class DescriptorPool(object):
       pass
 
     try:
-      return self._toplevel_extensions[symbol].file
+      return self._file_desc_by_toplevel_extension[symbol]
     except KeyError:
       pass